The invention relates to an optical prism assembly, and in particular to an optical prism assembly for a digital light processing (DLP) projection system.
FIG. 1 is a schematic view of a conventional digital light processing (DLP) projection system disclosed by U.S. Pat. No. 5,826,959, utilizing a cross dichroic prism emitting synthesized light of red (R), blue (B), and green (G) for projection. As shown in FIG. 1, the conventional DLP projection system comprises a cross dichroic prism 100, three polarization beam splitters 102, 104, and 106, and three spatial light modulators (SLM) 108, 110, and 112. When images are produced via the combination of the cross dichroic prism 100 and the spatial light modulators 108, 110, and 112, each spatial light modulator 102, 104, or 106 operates on one of the red, green, and blue light components. The spatial light modulators 108, 110, and 112 are referred to as three liquid crystal (LC) light valves 108, 110, and 112, respectively. The polarization beam splitters 102, 104, and 106 are disposed between the liquid crystal light valves 108, 110, and 112 and the cross dichroic prism 100, respectively. Two dichroic filters or mirrors 120 and 122 intersected in the cross dichroic prism 100 can reflect only blue or red light components and allow other colors therethrough. Thus, when the red, green, and blue light components pass through the polarization beam splitters 102, 104, and 106, respectively, reflected by the liquid crystal light valves 108, 110, and 112, the cross dichroic prism 100 combines the red, green, and blue light components and transmits the synthesized light for projection in a perpendicular direction.
The conventional design, however, has the following disadvantages. Each colored light has a large incident angle of 45° inclined at the dichroic filters 120 and 122. If the incident angle is too large, the polarization light spectrum through the dichroic filters is shifted. The overall efficiency of colored light in the prism assembly is reduced, accordingly, reducing color purity and brightness.